Initially following the basic 0.75 +/- 0.25 cups of glucose rule and later using 4 g/l sucrose formula, I have bottle-carbonated many brews to 2.2-3.5 volumes of CO2. Still, for whatever reason, I remain convinced that most lagers (megabrews especially) have significantly more "bite".

Could it be that Heineken, Stella or Sam Adams for example, have more than 3.5 volumes of CO2? Or, could it be that the perceived higher carbonation sensation is due to their dryness compared to my brews?

I would like to experiment with higher levels of CO2, but do not know how high is safe to go. What are your experiences? (I definitely plan to use plastic screw-top brown 0.5 l bottles for this, for safety reasons)

It's a little of both reasons you suggest, q-ceps. I wouldn't say that most European lagers are more highly carbonated than American light lagers (about 2.6 volumes of CO2), but certainly many other beers (Belgians, hefeweizens, etc.) are. The carbonation does make the beer seem "sharper" and more sour because some of the CO2 reacts with water to produce carbonic acid, which lowers the pH of the beer.

As for "safe" volumes of CO2, some sodas are carbonated to as many as 5 volumes of CO2, so I wouldn't worry too much about the safety of PET plastic bottles unless you get carried away with yourself. Glass is somewhat more dangerous, which is why champagne bottles are thicker than those used for still wines.